1. Field of the Invention
The present invention relates to an exhaust purification device of an internal combustion engine.
2. Description of the Related Art
A three-way catalyst has long been used for removing the harmful components from exhaust gas. Such a three-way catalyst has a so-called O.sub.2 storage function whereby it takes in and stores the oxygen in the exhaust gas when the air-fuel ratio becomes lean. It uses this O.sub.2 storage function to efficiently remove the three harmful components in exhaust gas, that is, HC, CO, and NOx. Accordingly, when this storage function weakens, the purification ability becomes lower, that is, the three-way catalyst deteriorates.
If the ratio between the total amount of air and the total amount of fuel fed to the exhaust passage upstream of a certain point in the exhaust passage or in the intake passage is defined as the air-fuel ratio of the exhaust gas at that certain point, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst changes from lean to rich, the unburnt HC and the CO in the exhaust gas seize the oxygen stored in the three-way catalyst. The unburnt HC and the CO can be oxidized by the seized oxygen. During the period when the unburnt HC and the CO are oxidized by the oxygen stored in the three-way catalyst in this way, the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst becomes substantially the stoichiometric air-fuel ratio. When the oxygen stored in the three-way catalyst runs out, the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst becomes rich. In this case, the stronger the O.sub.2 storage function, the longer the time when the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst is maintained at substantially the stoichiometric air-fuel ratio. When the O.sub.2 storage function weakens, the time during which the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst is maintained at substantially the stoichiometric air-fuel ratio becomes shorter.
There is known an internal combustion engine which is designed so that after the exhaust gas flowing into the three-way catalyst changes from lean to rich, the time during which the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst is maintained at substantially the stoichiometric air-fuel ratio is detected and it is judged that the degree of deterioration of the three-way catalyst is advancing along with the shortening of that time (see Japanese Unexamined Patent Publication (Kokai) No. 2-130245).
The time during which the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst is maintained at substantially the stoichiometric air-fuel ratio is, however, extremely short. It is difficult to accurately detect such a short time without error and to accurately judge the degree of deterioration from the results of that detection. Further, sometimes the air-fuel ratio of the exhaust gas flowing out from the three-way catalyst becomes rich in a spike-like fashion due to some reason or another when maintained at the stoichiometric air-fuel ratio. If such a "rich spike" occurs, there is the problem that the degree of deterioration of the three-way catalyst ends up being completely erroneously judged.